The present invention relates to bearing assemblies of the self-aligning type and, more particularly, to a method of assembling bearings which include a bearing surface made of a curable mixture such as an acrylate composition and a particulate solid lubricant.
In the art of constructing self-aligning bearings, wherein a truncated ball member which has a hollow core is retained in an outer bearing race member, the problem of assembling the bearing with the outer race member encompassing the ball member without destroying the structural integrity of the resulting bearing assembly has long been known.
Previously, it has been the practice to employ mechanical force to swage the outer bearing member about the ball surface. However, it is readily apparent that the amount of deformation that results is unpredictable in many instances and will result in non-uniform bearing surfaces which contributes to localized deterioration of these surfaces and thus premature failure of the bearing assembly. Also, as is well known, in order to employ mechanical deformation, one of the bearing elements must be made from a material that is deformable while the other bearing element must be constructed from a substantially harder material. As a result, the wear characteristics and durability of the resulting bearing are limited by the lower tensile characteristics of the softer material. This latter disadvantage is overcome to a degree by the use of injected plastic bearing surfaces, however, the problem of assembling the outer bearing rings about the ball member is not significantly alleviated by resort to such procedures since the necessity of deforming the outer race member to retain the ball member is still present. Moreover, where such plastic bearing surfaces are first injected or disposed between the inner and outer bearing members prior to deformation of the outer bearing element, the necessary subsequent swagging or deformation step can often result in the same unpredictable localized surface irregularities as were present in the metal to metal bearing assemblies.
Ideally, it is desirable to employ substantially identically hardened inner and outer bearing elements to thus maximize the durability of the bearing assembly. As a consequence, it has been proposed to employ finely machined and completely hardened outer bearing elements which are mechanically maintained in an assembled condition.
Such arrangements, however, suffer from the disadvantage that where the bearing assembly is subjected to high frequency vibration and/or a great range of environmental variations in terms of temperature or moisture such as are encountered in aircraft applications non-uniformities or misalignment in the bearing surfaces will result and thus cause premature bearing failure.
Where one of the bearing surfaces consists of a molded composition or a spray deposited brittle material, such non-uniformities in the alignment of the bearing elements is particularly disadvantageous since the bearing composition will be subjected to localized stresses which will invariably damage the bearing composition.
In some arrangements, such as disclosed in Reissue Pat. No. Re. 27,778 of Oct. 9, 1973, it has been proposed to secure the outer bearing members directly to a housing by fusion welding in an axial direction the outer surface of the bearing elements to a housing. It has been found to be impractical, however, for a manufacturer of bearing elements to employ fusion welding for every type of application of the bearing assembly. Additionally, particularly where a molded composition is employed as the bearing surface, the stress concentration and shrinkage which result from fusion welding can result in an unpredictable misalignment of the molded bearing surface with the inner ball member particularly where the outer welded member is being secured to an external housing.
The improvements of the present invention will provide useful solutions to a number of the foregoing problems recognized in the prior art as well as advantages which will enhance the production efficiency as well as extend the useful life of the resulting bearing assemblies.
According to the method of the present invention, in one embodiment, two ring members of substantially identical dimensions are provided with inner annular surfaces which, when the rings are joined together, form the outer bearing race surface which may be coated with a curable bearing surface composition. The ring members are provided with substantially planar mating surfaces, each of which is formed with an annular recess adjacent the inner edge of each ring. Thus, when the rings are placed in assembled condition, the recesses will define a circumferential cavity. With the ball member having been previously inserted in one of the ring members, the other ring member is placed in contact with the first ring member. The ring members are held together by a circumferential electron beam weld which is radially directed from the outer periphery of the ring members to the cavity defined by the abutting recesses in the mating surfaces. The provision of the cavity serves to define the limit of the depth of penetration of the electron beam weld and will control any splatter resulting from the heating of the metal. Also, provision of the circumferential cavity will reduce stress concentration by compensating for shrinkage in the assembled bearing elements. That is to say, the walls of the cavity will absorb any localized deformations resulting from the heat generated as a result of the fusion bonding so that stresses that would normally be set up in the elements will be absorbed by deformation of the cavity walls.
In another embodiment, one of the mating surfaces of the ring members is provided with an upstanding lip at its inner peripheral edge with the inner walls of the lip defining the inner wall of the recess and, in the assembled condition, the cavity. The mating surface of the other ring member is provided with a recess about its inner peripheral surface into which the upstanding lip of the other ring member will project to close the recess when the elements are assembled. The radial thickness of the lip is less than the radial width of the recess so that, in the assembled condition, an annular void will exist. Thus, not only is accurate alignment of the two ring members assured but also, a void is provided to absorb molten metal and prevent the molten metal from reaching the inner annular surfaces of the ring members during the welding operation so that the physical integrity of the inner annular surfaces will be preserved. This is particularly important where the inner annular surfaces are to be coated with a bearing composition which, preferably, consists of an acrylate composition mixed with a particulate solid lubricant such as polytetrafluoroethylene, silver powder, lead powder, molybdenum disulphide or fiberglass flock as described in U.S. Pat. No. 3,806,216, of Apr. 23, 1974, which is assigned to the same assignee as the present invention. While other types of moldable bearing compositions may be employed, the types described in the aforementioned U.S. Patent are particularly useful with the method of the present invention since the welding operation of the present invention assures the obtaining of the uniform and aligned inner annular surface for receiving the polymeric composition whereby the possibility of localized non-uniformities in the bearing surface are substantially minimized, if not entirely eliminated.